The Birth of the Bab is dated according to the Islamic lunar calendar, on the first day of Muharram, rather than fixed to a specific date on the Baha'i solar calendar Baha'u'llah later established, a distinction that gives it, along with the Birth of Baha'u'llah observed the following day on the lunar calendar, a different position relative to the Gregorian calendar each year, unlike the majority of Baha'i holy days, which fall on fixed dates within the nineteen month Baha'i solar year.
Because the two births fall on consecutive days of the Islamic lunar calendar, Baha'i communities generally observe them together as a linked pair of holy days, often called the Twin Birthdays, with joint celebrations reflecting the close theological relationship Baha'i teaching holds between the Bab and Baha'u'llah, the former understood as directly preparing the way for the latter's own mission.
The continued use of the lunar calendar for these two specific anniversaries, within a religion that otherwise organizes its ritual year around Baha'u'llah's own solar calendar, reflects their origin within a Perso-Islamic religious environment and preserves a direct calendrical link to that historical context even as the wider Baha'i Faith developed its own independent calendrical system.